Much effort is presently being made to develop efficient and robust means of extracting energy from renewable resources. Harnessing wave energy—in particular—presents a number of design challenges with respect to both the efficiency and robustness of systems fit for such a purpose.
Many wave energy converters (WECs) rely on a movable body, such as a pontoon, moving in one mode, such as heaving motion. In such wave energy converters, the movable body is connected to a second reference body, such as a second, larger pontoon, or some form of fixed position structure, and the movement of the moveable body relative to the reference body is harnessed and converted into energy. However, it is proven that such designs are not capable of capturing more than 50% of the wave energy. There is therefore a need to provide a wave energy converter that is capable of extracting energy from waves more efficiently.
One proposal for extracting energy from ocean waves is a wave-powered prime mover comprising a pair of pontoons connected to a central inertial barge, for example as seen in WO 99/28622. The pontoons are arranged symmetrically with respect to the tethered barge and are pivotally movable by waves relative to the barge. The pitching of the pontoons operates hydraulic pumps connected between each pontoon and the barge to convert the pitching motion into water pressure energy. Another proposal for a wave energy plant comprising two pontoons is based on the principle of Cockerell's raft and is described in WO 2008/135046. Two interhinged pontoons are connected by a hydraulic power take-off system. The pontoons heave up and down as waves pass and this causes them to pivot about the centre of the device, causing movement of the hydraulic actuator. Yet another proposal for a wave power apparatus is described in WO 00/17519 and known as a Pelamis machine. This device comprises an articulated chain-like structure made up of cylindrical members that can undergo relative rotational movement. While some of these and other proposals have been commercialised, all such WEC devices rely on a single mode of motion to convert energy from the waves and therefore suffer from the inefficiencies outlined above.
Many WEC systems deploy oil-hydraulic cylinders as the actuators in the energy extraction system. However, it will be appreciated that there are significant environmental risks associated with using oil hydraulics, particularly at sea, where inevitable leaks cause problems. Other WEC systems propose electrical linear generators, but at this time, these are at a very early stage of development. Some WEC systems overcome these identified drawbacks by implementing hydraulic actuators that utilise seawater as the energy transfer medium. However, seawater has significant potential to degrade and break down materials exposed to it (through both mechanical and chemical action), and so seawater hydraulic systems need to be able to withstand such forces.
Most water pumps on the market today are of open impeller, centrifugal design. This is a low cost solution to general purpose pumping. These pumps, however, have very low efficiency (approx 40%) and more importantly, cannot deliver high pressure heads unless multiple pumps are configured in series. The complexity of such arrangements make them clearly unattractive for implementation in a wave energy converter. High pressure pumps, such as those used in the desalination industry (Reverse Osmosis plants) are of multiple stage design and generally use positive displacement pistons and cylinders as opposed to open impellers. However, these pumps cannot handle raw seawater, as this would lead to rapid wear and component failure, and are therefore limited in their utility.
U.S. Pat. No. 6,140,712 describes a double-acting hose pump used in the context of a wave energy converter. The pump consists of a pair of hose pumps connected to a common outlet pipe. A disadvantage with such a pump is that the pressure that the flexible hoses can withstand will limit the pressure generated. In an environment where a high pressure output is desirable (such as in the context of a wave energy converter), this would be a serious limitation.
Another pump is described in UK Patent No. GB 2453670. The described pump is a double-acting pump used in the context of a wave energy converter. The pump consists of a double-acting, reciprocating piston designed to pump seawater via a pair of suction legs, each comprising a chamber, an inlet valve and an outlet valve, through to a common outlet pipe. However, while on the upstroke of the piston the body of fluid passing through one suction leg is accelerated, the return stroke sees the fluid body on this side of the pump lose acceleration, only to be accelerated again on the next upstroke. This occurs in the same manner at the other suction leg and the return strokes of the piston. It will be appreciated that expending energy re-accelerating the fluid body at the start of each second stroke results in energy loss and a significant deterioration in pumping performance. Furthermore, it should be noted that the volumes of the two chambers may not be equal, because the piston shaft passes through one chamber but not the other. This may be disadvantageous if continual flow and pressure is desired. Furthermore, the piston/dual chamber arrangement comprises a single integrated unit that would be difficult to repair, because the surface cooperating with the plunger seal is situated on the inside of the chamber. Furthermore, leakage in the piston seal would affect the fluid being pumped out of both chambers. As mentioned above, piston based pumps are of little utility for pumping seawater, because the piston needs high pressure, high velocity, high wear resistance piston seals or rings, and there is nothing on the market yet to achieve this economically.
It would be desirable to provide a wave energy conversion device and related systems that address all the issues cited above, and overcome the limitations of existing solutions.